Method and apparatus for determining printing exposure amount

Abstract

A photographic printer for making a trimming print has a zoom lens whose focal length can be changed in accordance with a trimming magnification. Exposure time is corrected based on a reciprocity law failure correction amount corresponding to the magnification of the zoom lens, whereby a density of the photoprint is corrected as to a change in a reciprocity law failure. According to a preferred embodiment, test prints are made respectively at a plurality of magnifications of the zoom lens. The densities of the finished test print are measured to form reciprocity law failure correction data automatically in the photographic printer so that the measured densities coincide with standard densities.

What is claimed is: 
     
       1. A method for determining a printing exposure amount for making a trimming print at a specified magnification M by changing a focal length of a zoom lens in a photographic printer, said method comprising the following steps: calculating an exposure time T for said trimming print by an equation   T=T.sub.0 ·[f.sup.2 (M)·(1+M·φ.sub.EN /φ.sub.EX).sup.2 ]/[f.sup.2 (M.sub.0)·(1+M.sub.0 ·φ.sub.EN /φ.sub.EX).sup.2 ]       where T 0  is a reference exposure time corresponding to a reference magnification M 0  of said zoom lens, f(M 0  ) is a lens f-number corresponding to said reference magnification M 0 , f(M) is a lens f-number corresponding to said specified magnification M, φ EN  is a diameter of an entrance pupil of said zoom lens, and φ EX  is a diameter of an exit pupil of said zoom lens;   obtaining a reciprocity law failure correction amount corresponding to said exposure time T based on data of a relationship between an exposure time and a reciprocity law failure correction amount of color paper; and   determining said printing exposure amount for an image frame to be printed in said photographic printer based on said obtained reciprocity law failure correction amount.   
     
     
       2. A method as defined in claim 1, wherein said data are formed by making test prints sequentially at different values of exposure time according to a plurality of magnifications of said zoom lens, measuring a density of each of said test prints by use of a densitometer incorporated in said photographic printer, and automatically calculating reciprocity law failure correction amounts corresponding to said values of exposure time in accordance with a difference between said measured density and a standard density. 
     
     
       3. A method as defined in claim 2, wherein said data are stored in a look-up table memory. 
     
     
       4. A method as defined in claim 1, wherein said data are obtained by making test prints from an ND film with a plurality of printing conditions in changing a reciprocity law failure correction amount so that each of said test prints is finished in three predetermined color densities under a plurality of different zoom lens magnification conditions and different exposure times for said trimming print. 
     
     
       5. A method as defined in claim 4, wherein a super-over-exposure film is used to obtain said data in addition to said ND film. 
     
     
       6. A method as defined in claim 1, wherein said data are stored in a look-up table memory. 
     
     
       7. A method as defined in claim 1, wherein said data are stored in a read-only memory (ROM). 
     
     
       8. An apparatus for determining a printing exposure amount for making a trimming print at a specified magnification M by changing a focal length of a zoom lens in a photographic printer, comprising: means for calculating an exposure time T for said trimming print according to an equation   T=T.sub.0 ·[f.sup.2 (M)·(1+M·φ.sub.EN /φ.sub.EX).sup.2 ]/[f.sup.2 (M.sub.0)·(1+M.sub.0 ·φ.sub.EN /φ.sub.EX).sup.2 ]       where T 0  is a reference exposure time corresponding to a reference exposure time corresponding to a reference magnification M 0  of the zoom lens, f(M 0 ) is a lens f-number corresponding to said reference magnification M 0 , f(M) is a lens f-number corresponding to said specified lens magnification M, φ EN  is a diameter of an entrance pupil of said zoom lens, and φ EX  is a diameter of an exit pupil of said lens;   means for outputting a reciprocity law failure correction amount corresponding to said exposure time T based on data constituting a relationship between an exposure time for said trimming print and a reciprocity law failure correction amount of color paper; and   means for determining said printing exposure amount for an image frame to be printed in said photographic printer based on said outputted reciprocity law failure correction amount.   
     
     
       9. An apparatus as defined in claim 8, wherein said reciprocity law failure correction outputting means comprises a look-up table memory. 
     
     
       10. An apparatus as defined in claim 8, wherein said reciprocity law failure correction amount outputting means comprises a read-only memory (ROM). 
     
     
       11. An apparatus as defined in claim 8, further comprising: a densitometer for measuring a density of said test print; and   calculating means, connected to said densitometer, for automatically calculating a reciprocity law failure correction amount corresponding to a difference between a measured density of said test print and a standard density so as to cause said reciprocity law failure correction amount outputting means to store said reciprocity law failure correction amount.   
     
     
       12. An apparatus as defined in claim 11, wherein said densitometer is disposed along a path of a processed print in said photographic printer.